One of the most common types of epilepsy - mesial temporal lobe epilepsy (MTLE) - is characterized by spontaneous and recurrent partial seizures. Furthermore, many patients with MTLE cannot control their seizures with current antiepileptic drugs. Novel approaches aimed at predicting and treating impending mesial temporal lobe seizures are therefore particularly important. Thus, the overall goal of the present proposal is to explore the mechanism of seizure generation in MTLE, thereby facilitating the discovery of candidate biomarkers of and more efficacious therapies for this disorder. Recent studies have suggested that a deficiency in glutamine synthetase in the hippocampus is implicated in the generation of seizures in MTLE (Eid et al. Lancet 2004; 363: 28-37). Using state-of-the art methods in continuous video intracranial EEG monitoring, microdialysis, immunogold electron microscopy, 13C- and 15N-isotope labeling, and tandem mass spectrometry, the role of the glutamine synthetase deficiency in the generation of seizures will be systematically explored in a newly developed laboratory model of MTLE. In Specific Aim 1, the relationship among hippocampal glutamine synthetase deficiency, intra- and extracellular glutamate concentrations, seizures, and hippocampal pathology will be assessed. The hypothesis is that a deficiency in glutamine synthetase leads to chronic high concentrations of glutamate in astrocytes and the extracellular space of the hippocampus with particularly high glutamate levels in individuals suffering from severe seizures and extensive brain damage. In Specific Aim 2, transient elevations in naturally occurring brain chemicals will be explored as potential triggers of spontaneous seizures in MTLE. The hypothesis is that such triggers cause a transient surge in extracellular glutamate in the hippocampus with spontaneous seizures as the ultimate consequence. Finally, in Specific Aim 3, the concept that restoration of glutamine synthetase in the epileptogenic hippocampus may be used as a cure of MTLE will be evaluated. The hypothesis is that restoration of glutamine synthetase in the hippocampus leads to cessation of seizures even in the presence of extensive brain pathology.